Isobaric mass labels

ABSTRACT

The present invention relates to a set of two or more mass labels, wherein each mass label comprises the formula:
 
X-L-M-Re
 
wherein X is a reporter moiety having an exact mass, L is a bond cleavable by collision in a mass spectrometer, M is a mass modifier, and Re is a) a reactive functionality for attaching the mass label to an analyte or b) the analyte, wherein each mass label in the set has an integer mass, wherein each mass label in the set has the same integer mass, and wherein the set comprises two or more subsets of mass labels, each subset comprising one, two or more mass labels, and wherein, when the subset comprises two or more mass labels, the exact mass of the reporter moiety X of each mass label in the subset is different from the exact mass of the reporter moiety X of the mass labels in the same subset and in all other subsets, and wherein each mass label is distinguishable by mass spectrometry.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Stage Application of International Patent Application No. PCT/EP2016/080536 filed on Dec. 9, 2016, which claims priority to GB Application No. 1521919.9, filed on Dec. 11, 2015, the contents of each of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to useful reactive labels for labelling peptides and to methods for using these reactive labels, to identify and quantify peptides particularly peptides derived from complex protein mixtures. These reactive labels are of particular value for the analysis of peptides by high resolution and high mass accuracy mass analysers such as orbitraps, time-of-flight and ion cyclotron resonance mass analysers.

BACKGROUND OF THE INVENTION

The study of biological systems and particularly the understanding of human disease is dependent on the ability to detect changes caused in biological systems by or in response to a disease. Such changes provide means of diagnosis and offer insights into the targets for therapeutic compounds such as vaccines and medicines. A wide range of biological molecules need to be measured quantitatively to understand disease processes including nucleic acids, proteins, steroids, sugars and lipids. In this context, the ability to quantitatively detect such biomolecules using mass spectrometers has provided considerable advances in their study and application to human and also to veterinary disease. The same advances have also occurred in environmental analysis and monitoring, and in food and beverage manufacturing. In particular the use of stable isotopes to provide synthetic quantitative references has been developed in isotope dilution mass spectrometry for monitoring of all classes of biomolecules. However, these methods have traditionally required an available synthetic standard, which is not always possible.

Recently, a range of chemical mass tags bearing heavy isotope substitutions have been developed to further improve the quantitative analysis of biomolecules by mass spectrometry. Depending on the tag design, members of tag sets are either isotopic having the same chemical structure but different absolute masses, or isobaric and isotopomeric, having both identical structure and absolute mass. Isotopic tags are typically used for quantification in MS mode whilst isobaric tags must be fragmented in MS/MS mode to release reporter fragments with a unique mass.

An early example of isotopic mass tags were the Isotope-Coded Affinity Tags (ICAT) (Gygi, S. P. et al., (1999) Nat Biotechnol, 17, 994-999). The ICAT reagents are a pair of mass tags bearing a differential incorporation of heavy isotopes in one (heavy) tag with no substitutions in the other (light) tag. Two samples are labelled with either the heavy or light tag and then mixed prior to analysis by LC-MS. A peptide present in both samples will give a pair of precursor ions with masses differing in proportion to the number of heavy isotope atomic substitutions.

The ICAT method also illustrates ‘sampling’ methods, which are useful as a way of reconciling the need to deal with small populations of peptides to reduce the complexity of the mass spectra generated while retaining sufficient information about the original sample to identify its components. The ‘isotope encoded affinity tags’ used in the ICAT procedure comprise a pair of biotin linker isotopes, which are reactive to thiols, for the capture peptides comprising cysteine. Typically 90 to 95% or proteins in a proteome will have at least one cysteine-containing peptide and typically cysteine-containing peptides represent about 1 in 10 peptides overall so analysis of cysteine-containing peptides greatly reduces sample complexity without losing significant information about the sample. Thus, in the ICAT method, a sample of protein from one source is reacted with a ‘light’ isotope biotin linker while a sample of protein from a second source is reacted with a ‘heavy’ isotope biotin linker, which is typically 4 to 8 Daltons heavier than the light isotope. The two samples are then pooled and cleaved with an endopeptidase. The biotinylated cysteine-containing peptides can then be isolated on avidinated beads for subsequent analysis by mass spectrometry. The two samples can be compared quantitatively: corresponding peptide pairs act as reciprocal standards allowing their ratios to be quantified. The ICAT sampling procedure produces a mixture of peptides that still accurately represents the source sample while being less complex than MudPIT, but large numbers of peptides are still isolated and their analysis by LC-MS/MS generates complex spectra. With 2 ICAT tags, the number of peptide ions in the mass spectrum is doubled compared to a label-free analysis.

Further examples of isotopic tags include the ICPL reagents that provide up to four different reagents, and with ICPL the number of peptide ions in the mass spectrum is quadrupled compared to a label-free analysis. For this reason, it is unlikely to be practical to develop very high levels of multiplexing with simple heavy isotope tag design.

Whilst isotopic tags allow quantification in proteomic studies and assist with experimental reproducibility, this is achieved at the cost of increasing the complexity of the mass spectrum. To overcome this limitation, and to take advantage of greater specificity of tandem mass spectrometry isobaric mass tags were developed. Since their introduction in 2000 (WO01/68664), isobaric mass tags have provided improved means of proteomic expression profiling by universal labelling of amines and other reactive functions in proteins and peptides prior to mixing and simultaneous analysis of multiple samples. Because the tags are isobaric, having the same mass, they do not increase the complexity of the mass spectrum since all precursors of the same peptide will appear at exactly the same point in the chromatographic separation and have the same aggregate mass. Only when the molecules are fragmented prior to tandem mass spectrometry are unique mass reporters released, thereby allowing the relative or absolute amount of the peptide present in each of the original samples to be determined.

WO01/68664 sets out the underlying principles of isobaric mass tags and provides specific examples of suitable tags wherein different specific atoms within the molecules are substituted with heavy isotope forms including ¹³C and ¹⁵N respectively. WO01/68664 further describes the use of offset masses to make multiple isobaric sets to increase the overall multiplexing rates available without unduly increasing the size of the individual tags.

WO2007/012849 describes further sets of isobaric mass tags including 3-[2-(2,6-Dimethyl-piperidin-1-yl)-acetylamino]-propanoic acid-(2,5-dioxo-pyrrolidine-1-yl)-ester (DMPip-βAla-OSu).

Recently, with dramatic improvements in mass accuracy and mass resolution enabled by high mass resolution mass spectrometers such as the Orbitrap (Hu, Q. et al., (2005) J Mass Spectrom, 40, 430-443 & Makarov, A. (2000) Anal Chem, 72, 1156-1162), Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometers (Marshall, A. G. et al., (1998) Mass Spectrom Rev, 17, 1-35) and high resolution Time-of-Flight (TOF) mass spectrometers (Andrews, G. L. et al., (2011) Anal Chem, 83, 5442-5446), it has become possible to resolve millidalton differences between ion mass-to-charge ratios. This high resolution capability has been exploited to increase multiplexing of Isobaric Tandem Mass Tags using heavy nucleon substitutions of ¹³C for ¹⁵N in the reporter region which results in 6.32 millidalton differences between the respective reporter fragments upon analysis by MS/MS (McAlister, G. C. et al., (2012) Anal Chem, 84, 7469-7478 & Werner, T. et al., (2012) Anal Chem, 84, 7188-7194). Similarly, it has been shown that metabolic labelling with lysine isotopes comprising millidalton mass differences can be resolved by high-resolution mass spectrometry enabling multiplexing and relative quantification of samples in yeast (Hebert, A. S. et al., (2013) Nat Methods, 10, 332-334).

Despite the significant benefits of previously disclosed isobaric mass tags, the multiplexing rate has been limited to 10-plex in commercial reagents to date. In addition, tags comprising very small mass differences would be useful because labelled ions that are related to each other, e.g. corresponding peptides from different samples, would cluster closely in the same ion envelope with very distinctive and unnatural isotope patterns that would be readily recognisable and which will be much less likely to interfere with the identification of other different peptides.

Hence, there still remains the need for sets of tags, where each tag differs from the others by millidalton mass differences, for labelling peptides and biomolecules with multiplexing rates greatly in excess of 10-fold.

BRIEF DESCRIPTION OF THE INVENTION

In a first aspect, the present invention relates to a set of two or more mass labels, wherein each mass label comprises the formula: X-L-M-Re wherein:

-   -   X is a reporter moiety having an exact mass,     -   L is a bond cleavable by collision in a mass spectrometer,     -   M is a mass modifier, and     -   Re is a) a reactive functionality for attaching the mass label         to an analyte or b) the analyte, wherein each mass label in the         set has an integer mass, wherein each mass label in the set has         the same integer mass, wherein the set comprises two or more         subsets of mass labels, each subset comprising one, two or more         mass labels, wherein, when the subset comprises two or more mass         labels, the exact mass of the reporter moiety X of each mass         label in the subset is different from the exact mass of the         reporter moiety X of the mass labels in the same subset and in         all other subsets, wherein each mass label is distinguishable by         mass spectrometry, wherein each mass label has a reporter moiety         X comprising the following general formula:

wherein R¹ is H, a substituted or unsubstituted straight or branched C₁-C₁₀ alkyl group, or a structure selected from methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, neopentyl, tert-pentyl, iso-pentyl, sec-pentyl and 3-pentyl.

In another aspect, the present invention relates to a set of two or more mass labels, wherein each label comprises the formula: X-L-M-Re wherein X is a reporter moiety having an exact mass, L is a bond cleavable by collision in a mass spectrometer, M is a mass modifier, and Re is a reactive functionality for attaching the mass label to an analyte or the analyte, and X comprises the following general formula:

wherein R¹ is H, a substituted or unsubstituted straight or branched C₁-C₁₀ alkyl group, or a structure selected from methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, neopentyl, tert-pentyl, iso-pentyl, sec-pentyl and 3-pentyl.

In another aspect, the present invention relates to an array of mass labels, comprising two or more sets of mass labels according to the invention.

In another aspect, the present invention relates to a method of mass spectrometry analysis, which method comprises detecting an analyte by identifying by mass spectrometry a mass label or combination of mass labels relatable to the analyte, wherein the mass label is a mass label from a set or array of mass labels according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic of the 2-propyl-pyrrolidine-1-yl acetic acid reporter of this invention.

DETAILED DESCRIPTION OF THE INVENTION

Sets of Mass Labels

The present invention provides sets of isotopomeric reactive tags for the purposes of labelling peptides and other biomolecules with multiplexing rates greatly in excess of 10-plex. Co-selectable isotopologue arrays of isotomoperic reactive tags have masses differences in the range of millidalton which supports even higher levels of multiplexing.

The present invention also provides for methods of use of co-selectable isotopologue arrays of isotopomeric reactive tags that enable novel forms of analysis of labelled peptides, proteins and other biological molecules, particularly for the discovery of biologically significant differences between sets of biological samples.

In a first aspect, the present invention relates to a set of two or more mass labels, hereinafter referred to as “the first set of mass labels of the invention”, wherein each mass label comprises the formula: X-L-M-Re wherein:

-   -   X is a reporter moiety having an exact mass,     -   L is a bond cleavable by collision in a mass spectrometer,     -   M is a mass modifier, and     -   Re is a) a reactive functionality for attaching the mass label         to an analyte or b) the analyte, wherein each mass label in the         set has an integer mass, wherein each mass label in the set has         the same integer mass, wherein the set comprises two or more         subsets of mass labels, each subset comprising one, two or more         mass labels, wherein, when the subset comprises two or more mass         labels, the exact mass of the reporter moiety X of each mass         label in the subset is different from the exact mass of the         reporter moiety X of the mass labels in the same subset and in         all other subsets, wherein each mass label is distinguishable by         mass spectrometry, wherein each mass label has a reporter moiety         X comprising the following general formula:

wherein R¹ is H, a substituted or unsubstituted straight or branched C₁-C₁₀ alkyl group, or a structure selected from methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, neopentyl, tert-pentyl, iso-pentyl, sec-pentyl and 3-pentyl.

The term “exact mass” refers to the theoretical mass of the mass label or of the reporter moiety and is the sum of the exact masses of the individual isotopes of the entire mass label or reporter moiety, e.g. ¹²C=12.000000, ¹³C=13.003355 H¹=1.007825, ¹⁶O=15.994915. The “exact mass” takes account of mass defects.

The term “integer mass” is the sum of the integer masses of each isotope of each nucleus that comprises the molecule, e.g. ¹²C=12, ¹³C=13, ¹H=1, ¹⁶O=16. The integer mass of an isotope is the sum of protons and neutrons that make up the nucleus of the isotope, i.e. ¹²C comprises 6 protons and 6 neutrons while ¹³C comprises 6 protons and 7 neutrons. This is often also referred to as the nominal mass, or atomic mass number or nucleon number of an isotope.

In the literature the term “isobaric” often refers to species that have the same integer mass and are co-selectable for MS/MS but in the context of this invention we will use the term “isobaric” refer to species that have the same exact mass and we will use the term “pseudo-isobaric” for species that have the same integer mass but may have slightly differing exact masses.

The difference in exact mass between at least two of the mass labels in a subset is usually less than 100 millidaltons, preferably less than 50 millidaltons, most preferably less than 20 millidaltons (mDa). Preferably, the difference in exact mass between at least two of the mass labels in a set is 2.5 mDa, 2.9 mDa, 6.3 mDa, 8.3 mDa, 9.3 mDa, or 10.2 mDa due to common isotope substitutions. For example, if a first label comprises a ¹³C isotope, and in a second label this ¹³C isotope is replaced by ¹²C but a ¹⁴N isotope is replaced by a ¹⁵N isotope, the difference in exact mass between the two labels will be 6.3 mDa.

In the present specification, the term label is synonymous with the term tag.

The term “reporter moiety X” is used to refer to a moiety of the mass label to be detected independently, typically after cleavage, by mass spectrometry, however, it will be understood that the remainder of the mass label attached to the analyte as a complement ion may also be detected in methods of the invention. The mass modifier X is a moiety which is incorporated into the mass label to ensure that the mass label has a desired integer mass. The reporter moiety X of each mass label may in some embodiments comprise no heavy isotopes.

The components of the reporter moiety according to the invention are preferably fragmentation resistant so that the site of fragmentation of the reporter moiety can be controlled by the introduction of a cleavable bond L that is easily broken by Collision Induced Dissociation (CID), Surface Induced Dissociation, Electron Capture Dissociation (ECD), Electron Transfer Dissociation (ETD), or Fast Atom Bombardment. In the most preferred embodiment, the linkage is easily broken by CID.

It will be understood by the person skilled in the art that in order to achieve the desired integer masses, one or both of the moieties X and M, the reactive functionality Re or the analyte may be modified with heavy isotopes. Typically the heavy isotopes are selected from ²H, ¹³C, ¹⁵N or ¹⁸O.

Preferably, the reporter moiety of each mass label in a subset is an isotopologue of the reporter moiety of all other mass labels in the subset. Isotopologues are chemical species that differ only in the isotopic composition of their molecules. For example, water has three hydrogen-related isotopologues: HOH, HOD and DOD, where D stands for deuterium (²H). Isotopologues are distinguished from isotopomers (isotopic isomers) which are isotopic isomers having the same number of each isotope but in different positions. More preferably, the set of two or more mass labels comprises at least one subset comprising two or more mass labels.

Usually, the difference in exact mass is provided by a different number or type of heavy isotope substitution(s).

In one embodiment, the mass labels are isotopologues of Tandem Mass Tags as defined in WO01/68664.

In a preferred embodiment the aggregate molecular weight of the mass label is 600 Daltons or less, more preferably 500 Daltons or less, still more preferably 400 Daltons or less, most preferably from 300 to 500 Daltons.

In another preferred embodiment, the molecular weight of the reporter moiety is 400 Daltons or less, preferably 250 Daltons or less, more preferably 100 to 250 Daltons, most preferably 100-220 Daltons. A reporter moiety of small size is particularly advantageous because it produces a peak in the silent region of a mass spectrum, which allows the reporter moiety to be easily identified from the mass spectrum and also allows sensitive quantification.

The term silent region of a mass spectrum used in the present context is intended to refer to the region of a mass spectrum with low background “noise” caused by peaks relating to the presence of fragments generated by fragmentation of the labelled peptides. Thus, the term silent region is intended to refer to the region of the mass spectrum with low “noise” caused by peaks relating to the peptide to be detected. For a peptide or protein, the silent region of the mass spectrum is less than 220, preferably less than 200 Daltons.

The mass labels according to the invention are designed to be reacted with a biomolecule, such as a protein to form a labelled biomolecule, e.g. a labelled protein.

In one embodiment, R¹ is methyl. In another embodiment, R¹ is propyl. In another embodiment, R¹ is isopropyl. In another embodiment, R¹ is butyl. In another embodiment, R¹ is isobutyl.

In a preferred embodiment, the reporter moiety X comprises an isotopic mass adjuster moiety in at least one of the atoms. The isotopic mass adjuster moiety may be ¹⁸O, ¹³C, ¹⁵N or ²H and one or more may be present.

In another embodiment, the cleavable bond L comprises, without limitation, an amide bond, a urea linkage, an ester linkage or an ether linkage. In a preferred embodiment, cleavable bond L comprises an amide bond. In another preferred embodiment, the cleavable bond L comprises a urea linkage. In another preferred embodiment, the cleavable bond L comprises an ester linkage. In another preferred embodiment, the cleavable bond L comprises an ether bond.

The term “mass modifier M”, as used herein, refers to a moiety that ensures that each mass label in the set has a desired integer mass. The mass modifier M is not necessarily to be detected by mass spectrometry. However, the mass modifier M may be detected as part of a complement ion (see below). The mass modifier M is not particularly limited structurally, but merely serves to vary the overall mass of the mass label.

In another embodiment, the mass modifier M has the following structure:

wherein:

-   -   each R¹⁰ is independently H, a substituted or unsubstituted         straight or branched C₁-C₆ alkyl group, a substituted or         unsubstituted aliphatic cyclic group, a substituted or         unsubstituted aromatic group or a substituted or unsubstituted         heterocyclic group or an amino acid side chain,     -   a is an integer from 0-10,     -   b is at least 1, and     -   c is at least 1.

In a preferred embodiment, the mass modifier M is selected from:

wherein * is an isotopic mass adjuster moiety and represents that oxygen is ¹⁸O, carbon is ¹³C, nitrogen is ¹⁵N or hydrogen is ²H, and wherein one or more * may be present.

In another embodiment, each mass label additionally comprises at least one mass series modifying group, wherein the mass series modifying group is part of the reporter moiety X and/or part of the mass modifier M.

Preferably, each mass label comprises a mass series modifying group, wherein the at least one mass series modifying group is part of the reporter moiety X or of the mass modifier M or both. More preferably, the mass series modifying group is part of the reporter moiety X.

Preferably, the mass series modifying group may be selected from:

-   -   a) a heavy isotope ²H, ¹³C, ¹⁵N or ¹⁸O;     -   b) a substituted or unsubstituted straight or branched C₁-C₁₀         alkyl group optionally comprising one or more heavy isotope         substitutions;     -   c) or a combination of a) and b).

In one embodiment, the mass series modifying group is selected from —CH₃, —¹³CH₃, —CHD₂, —¹³CHD₂, —¹³CD₃ or —CD₃.

In another preferred embodiment, each mass label comprises at least one mass series modifying group having the following structure:

wherein:

-   -   each R¹¹ is independently H, a substituted or unsubstituted         straight or branched C₁-C₆ alkyl group, a substituted or         unsubstituted aliphatic cyclic group, a substituted or         unsubstituted aromatic group or a substituted or unsubstituted         heterocyclic group or an amino acid side chain;     -   d is an integer from 1 to 10;     -   e is an integer from 1 to 10; and     -   f is an integer from 1 to 10.

In the mass labels according to the invention, Re may either be a reactive functionality for attaching the mass label to an analyte or be an analyte.

Preferably the mass tags additionally comprise a reactive functionality to allow the mass label to be conjugated to an analyte. The reactive functionality for attaching the mass label to the analyte is not especially limited and may comprise any appropriate reactive group.

The reactive functionality may react with an amino group on the biological molecule, for example the ε-amino group of a lysine residue. In the simplest embodiments this may be an N-hydroxysuccinimide ester. Other reactive functionalities are contemplated herein such as those which react with thiol groups in biological molecules. In particular these reactive functionalities are designed to react with the thiol group of a cysteine residue. Examples of reactive groups of the present invention which are able to react with cysteine residues are the maleimido, haloacetyl and 2-dithiopyridine groups. The thiol group of cysteine undergoes nucleophilic addition across the double bond of the maleimido group and undergoes nucleophilic substitution with the haloacetyl or 2-dithiopyridine group.

Reactive functionalities which are capable of reacting with carbonyl or hydroxyl groups in biological molecules are also contemplated herein. In particular, these reactive functionalities are designed to react with the carbonyl or hydroxyl groups of steroid hormones. Reactive groups of the present invention which are able to react with carbonyl or hydroxyl groups in a biological molecule are hydrazide or —CONH—(CH₂)^(n)—ONH₂, wherein n is from 1 to 6, and preferably n is 3 i.e. aminoxypropyl amide. These groups react with carbonyl groups to form hydrazones or O-alkyloximes respectively. Examples of reactive functionalities are shown in WO2011/036059, which reference is incorporated herein.

Preferably, the reactive functionality is an N-hydroxysuccinimide ester, a 2,3,5,6-tetrafluorophenyl ester or a sulphodichlorophenyl ester.

When Re is the analyte, the analyte preferably comprises amino acids, peptides, polypeptides, nucleotides, oligonucleotides, polynucleotides, carbohydrates, lipids, phospholipids or combination thereof.

Improving multiplexing is a highly sought characteristic of isobaric mass labels as it allows labelling of a high number of sample and analysis is one single experiments thus reducing time of analysis, costs and also standardizing the analysis conditions for a higher number of samples. In order to generate mass labels for isobaric mass labelling using only ¹⁵N and ¹³C substitutions in a mass label according to the general structures disclosed in the present invention, it is necessary to consider the positions substitutable with heavy isotope mass series modifying groups comprising 2 different elements (P positions) and positions substitutable for the first element (A positions) and positions substitutable for the second element (B positions) different from the first. The number of A positions should be greater than or equal to the number of B positions. Assuming there are (P+1) subsets of mass labels and the x^(th) subset of mass labels comprises C mass labels, C should be less than or equal to (B+1). Each reporter moiety comprises (x−1) positions substituted with heavy isotopes from either the first or second element and where the w^(th) mass label in each subset of mass labels comprises y atoms of the first heavy isotope element and z atoms of the second heavy isotope element different from the first, x will have values from 1 to (P+1). P=(A+B) and the total number of mass labels will be (A+1) multiplied by (B+1). Typically, the heavy isotopes are selected from ²H, ¹³C and ¹⁵N.

In preferred embodiments B is greater than or equal to 2.

For example, a mass label where there are 8 dopable carbons and 1 dopable Nitrogen in the reporter moiety and in the mass modifier, will support up to 18-plex isobaric sets, i.e. (8+1) multiplied by (1+1). At single Dalton resolution, these reporters will support 10-plex (P=8+1) giving (9+1) subsets of mass labels with different integer reporter masses. Obviously as the reporter moiety groups can be substituted with different R-groups different isomers of the mass labels are possible, providing options for different fragmentation behaviours.

The most preferred mass labels according to the invention together with examples of sets of two or more mass labels comprising heavy isotopes mass series modifying groups are described in details herein below in preferred Embodiments 1 to 4. The mass labels are identified by the set number, parent set size and the reporter ion mass, e.g. in Embodiment or set 1 or below, each mass label is named TMT-1-18-“reporter mass”, where TMT stands for Tandem Mass Tag, i.e. tags for tandem mass spectrometry, the digit 1 refers to the Set number, the 18 refers to the number of mass labels in the set and the reporter mass is the mass-to-charge ratio of the expected reporter ion under Collision Induced Dissociation conditions. Different reporter ions may be obtained by Electron Transfer Dissociation (ETD) or Electron Capture Dissociation (ECD).

Embodiment 1

The mass label has structure:

wherein * represents that oxygen is ¹⁸O, carbon is ¹³C, nitrogen is ¹⁵N or hydrogen is ²H, and wherein one or more * may be present.

In a specific preferred embodiment of an isobaric set of mass tags according to this invention, the mass adjuster moiety * is ¹³C or ¹⁵N and the set comprises n=18 mass labels having the following structures:

Taking the terminology set out in the preferred embodiment described above, m (as defined above) is 18 and n=9. Since there are more carbon atoms into which ¹³C can be substituted than nitrogen atoms into which ¹⁵N can be substituted, there are a=8 substitutable Carbon nuclei and b=1 substitutable Nitrogen nuclei. Thus there are 8 atoms of the first heavy isotope mass adjuster, which is ¹³C, incorporated into each tag and 1 atom of the second heavy isotope mass adjuster, which is ¹⁵N and the full set of mass tags is created by making all the possible combinations of mass adjusters on either side of the fragmentable bond, which is marked with the dashed line in the structures above. It can be seen in the list above that there are (n+1)=10 subsets of tags based on the integer mass of the reporter ions, i.e. the reporter ions in subset 2 are approximately 1 dalton heavier than the reporter ions in subset 1. Similarly, the reporter ions in subset 3 are approximately 1 dalton heavier than the reporter ions in subset 2, etc. Within each subset of tags, it can be seen from the calculated exact masses that each tag differs from the next by 6.32 Millidaltons. In subset 1, there are no heavy isotope mass adjusters in the reporter ion and there is only one way in which this reporter can be constructed so there is only 1 tag in subset 1. In subset 2, there is one heavy isotope mass adjuster in the reporter ion, shifting the mass of the reporter by approximately 1 dalton relative to subset 1. There are 2 ways to introduce the mass adjuster, by introduction of a single ¹⁵N nucleus or by introduction of a single ¹³C nucleus and hence there are two tags in subset 2 differing in mass from each other by 6.3 millidaltons. In subset 3, there are two heavy isotope mass adjusters in the reporter ion, shifting the mass of the reporter by approximately 1 dalton relative to subset 2. There are 2 ways to introduce the 2 mass adjusters into subset 3, by introduction of a single ¹⁵N nucleus and a single ¹³C nucleus or by introduction of two ¹³C nuclei and hence there are 2 tags in subset 3. In subset 4, there are three heavy isotope mass adjusters in the reporter ion, shifting the mass of the reporter by approximately 1 dalton relative to subset 3. There are again only 2 ways to introduce the 3 mass adjusters into subset 3, by introduction of a single ¹⁵N nucleus and a two ¹³C nuclei or by introduction of three ¹³C nuclei and hence there are 2 tags in subset 4. In general, the number of tags in each subset is limited by which of the mass adjuster nuclei is present less frequently in the structure. In example set 1, there is only one nitrogen nucleus in the reporter and in the mass normalizer and so b=1 as defined above and the number of tags in each subset of tags <=(b+1), which is a maximum of 2 tags per subset. In the 9^(th) subset, there are 8 heavy isotope mass modifiers and there are only two ways to construct a reporter ion with the 8 heavy isotopes while retaining an overall isobaric tag structure so there are only two tags in the 9^(th) subset and similarly in the 10^(th) subset, all 9 heavy isotope mass adjusters are present in the reporter and there is only one way to construct a reporter with all of the mass modifiers, so there is only 1 tag in subset 10.

It should be clear to one of ordinary skill in the art that the mass normalizer group, which comprises two beta-alanine residues in this tag, could be varied considerably. Obvious substitutions include, replacement with other amino acids such as alanine, valine, leucine or with longer amino acids such as gamma-aminobutyric acid, aminopentanoic acid or aminohexaminoic acid. Poly-ethylene glycol linkers might also be appropriate with an amino and a carboxylic acid terminus.

Embodiment 2

The mass label has structure:

wherein * represents that oxygen is ¹⁸O, carbon is ¹³C, nitrogen is ¹⁵N or hydrogen is ²H, and wherein one or more * may be present.

An example of a set of n=16 mass labels comprising the mass series modifying groups ²H (i.e. D)¹³C or ¹⁵N is shown below:

The person skilled in the art will immediately appreciate that although the fixed substitutions of ²H, ¹³C and ¹⁵N are shown in a particular location in the examples shown above, this has been done as a convenience for the purposes of explanation and these fixed substitutions in Embodiment Set 2 could located at any suitable location within the reporter ion if it is more convenient or cost-effective to locate them elsewhere.

Embodiment 3

The mass label has structure:

wherein * represents that oxygen is ¹⁸O carbon is ¹³C, nitrogen is ¹⁵N or hydrogen is ²H, and wherein one or more * may be present.

It is immediately apparent that a fixed substitution of 5 deuterium nuclei is present in this tag structure.

An example of a set of n=18 mass labels comprising the mass series modifying groups ¹³C or ¹⁵N is shown below:

It would be apparent that that the tags in Tag Set 3 are all isotopes of the tags in Tag Set 1 and each tag is just over 5 daltons heavier than the corresponding tags in Tag Set 1. Moreover, all the reporters in Tag Set 3 are uniquely resolvable from the tags in Tag Set 1. These sets of tags could thus be used simultaneously to support 36-plex multiplexing. Note that peptides labelled with tags from Tag Set 3 are not isobaric with the tags of Tag Set 1; the tags in Tag set 3 would be offset from the tags in Tag set 1 by 5 daltons.

Embodiment 4

The mass label has the structure:

wherein * represents that oxygen is ¹⁸O, carbon is ¹³C, nitrogen is ¹⁵N or hydrogen is ²H, and wherein one or more * may be present.

It is immediately apparent that a fixed substitution of 5 deuterium nuclei is present in this tag structure.

In a specific preferred embodiment of an isobaric set of mass tags according to this invention, the mass adjuster moiety * is ¹³C or ¹⁵N and the set comprises n=7 mass labels having the following structures:

It should be apparent that that the 7 tags in Tag Set 4 are all isotopes of the tags in Tag Set 1 and each tag is approximately isobaric with the tags in Tag Set 1. Moreover, all the reporters in Tag Set 4 are uniquely resolvable from the tags in Tag Set 1. These sets of tags could thus be used simultaneously to support 25-plex multiplexing. Peptides labelled with tags from Tag Set 4 are isobaric with and co-selectable with corresponding peptides labelled with the tags of Tag set 1 hence the tags in Tag set 4 and the tags in Tag set 1 would together form a larger pseudo-isobaric array of tags.

In another aspect, the present invention relates to a set of two or more mass labels, hereinafter referred to as “the second set of mass labels of the invention”, wherein each label comprises the formula: X-L-M-Re wherein X is a reporter moiety having an exact mass, L is a bond cleavable by collision in a mass spectrometer, M is a mass modifier, and Re is a reactive functionality for attaching the mass label to an analyte or the analyte, and X comprises the following general formula:

wherein R¹ is H, a substituted or unsubstituted straight or branched C₁-C₁₀ alkyl group, or a structure selected from methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, neopentyl, tert-pentyl, iso-pentyl, sec-pentyl and 3-pentyl.

The definitions and particular and preferred embodiments of the first set of mass labels of the invention are equally applicable to the second set of mass labels of the invention.

The individual mass labels described previously in the context of the first and the second sets of mass labels of the invention constitute additional aspects of the present invention.

Arrays of Mass Labels

The present invention also provides an array of mass labels, hereinafter referred to as “the array of mass labels of the invention”, comprising two or more sets of mass labels according to the first and the second sets of mass labels of the invention.

The definitions and particular and preferred embodiments described in detail in the context of the first set of mass labels of the invention apply equally to the array of mass labels of the invention.

In an embodiment, the integer mass of each of the mass labels of any one set in the array is different from the integer mass of each of the mass labels of every other set in the array.

In a preferred embodiment, each mass label in a set comprises:

-   -   a) a mass series modifying group having the same integer mass as         every other mass label in the set, and     -   b) a different integer mass to the mass labels of all the other         sets of the array.

In a particularly preferred embodiment the reporter moiety X comprises the mass series modifying group.

In one embodiment each mass label in a set comprises the same mass series modifying group.

In another embodiment, each mass label in a set comprises a mass series modifying group which is:

-   -   a) the same; or     -   b) an isotopologue of the mass series modifying group of all         other mass labels of the array.

In a preferred embodiment, each mass label in a set comprises a mass series modifying group which is an isotopologue of the mass series modifying group of all other mass labels of the array.

Methods of Mass Spectrometry Analysis

The present invention also provides for a method of mass spectrometry analysis, hereinafter referred to as “the method of mass spectrometry analysis of the invention”, which method comprises detecting an analyte by identifying by mass spectrometry a mass label or combination of mass labels relatable to the analyte, wherein the mass label is a mass label from the first or the second set of mass labels of the invention, or the array of mass labels of the invention, as defined in the previous aspects of the present invention.

In one embodiment, the method of mass spectrometry analysis of the invention comprises:

-   -   a) providing a plurality of samples, wherein each sample is         differentially labelled with a mass label or a combination of         mass labels, wherein the mass label(s) are from the first or the         second set of mass labels of the invention, or the array of mass         labels of the invention;     -   b) mixing the plurality of labelled samples to form an analysis         mixture comprising labelled analytes;     -   c) optionally detecting the labelled analytes in a mass         spectrometer;     -   d) dissociating the labelled analytes in the mass spectrometer         to form mass labels and/or analyte fragments comprising intact         mass labels;     -   e) detecting the mass labels and/or analyte fragments comprising         intact mass labels;     -   f) optionally dissociating the mass labels in the mass         spectrometer to release the reporter moieties, and detecting the         reporter moieties;     -   g) optionally dissociating the reporter moieties formed in         step f) to form fragments, and detecting the fragments;     -   h) identifying the analytes on the basis of the mass spectrum of         the labelled analytes;

and/or the mass spectrum of the mass labels and/or analyte fragments comprising an intact mass label; and/or the mass spectrum of the reporter moieties or fragments of reporter moieties.

In a particular embodiment, the dissociation is, preferably, collision induced dissociation in a mass spectrometer.

In another particular embodiment, a complement ion is formed in step d) by neutral loss of carbon monoxide from the linker L.

Preferably the methods described herein may be performed in a mass spectrometer with a resolution of greater than 60,000 at a mass-to-charge ratio of 400, preferably a resolution of greater than 100,000 at a mass-to-charge ratio of 400, most preferably greater than 250,000 at a mass-to-charge ratio of 400.

The analytes may be identified on the basis of i) the mass spectrum of the labelled analytes; or ii) the mass spectrum of the mass labels and/or analyte fragments comprising an intact mass label; or iii the mass spectrum of the reporter moieties or fragments of reporter moieties. When identification according to ii) occurs, the analyte fragment preferably comprises an intact mass label is a b-series ion comprising an intact mass label, preferably a b1 ion. The analytes may be identified on the basis of the mass spectrum of the reporter moieties X or fragments of reporter moieties X.

Thus, in one embodiment, the analytes may be identified on the basis of the mass spectrum of the labelled analytes.

In another embodiment, the analytes may be identified on the basis of the mass spectrum of the mass labels and/or analyte fragments comprising an intact mass label. In a preferred embodiment, the analyte fragment comprising an intact mass label is a b-series ion comprising an intact mass label, preferably a b1 ion.

In another embodiment, the method of mass spectrometry analysis of the invention comprises:

-   -   a) providing a plurality of samples, wherein each sample is         differentially labelled with a mass label or a combination of         mass labels, wherein the mass label(s) are from the first or the         second set of mass labels of the invention, or the array of mass         labels of the invention;     -   b) mixing the plurality of labelled samples to form an analysis         mixture comprising labelled analytes;     -   c) detecting the labelled analytes in a mass spectrometer;     -   d) dissociating the labelled analytes in the mass spectrometer         to release the reporter moieties, and detecting the complement         ions comprising the remainder of the mass label attached to the         analyte or a fragment of the analyte;     -   e) optionally one or more further steps of dissociating the         complement ions formed in step d to form fragments, and         detecting the fragments;     -   f) identifying the analytes on the basis of the mass spectrum of         the labelled analytes and/or the mass spectrum of the complement         ions and/or fragments thereof.

In a particular embodiment, the dissociation is, preferably, collision induced dissociation in a mass spectrometer.

In another particular embodiment, a complement ion is formed in step d) by neutral loss of carbon monoxide from the linker L.

Preferably the methods described herein may be performed in a mass spectrometer with a resolution of greater than 60,000 at a mass-to-charge ratio of 400, preferably a resolution of greater than 100,000 at a mass-to-charge ratio of 400, most preferably greater than 250,000 at a mass-to-charge ratio of 400.

Many of the mass labels of this invention are differentiated from each other by very small mass difference, sometimes of the order of only 1 millidalton. It has already be established that current Orbitrap instrumentation can resolve reporter ions with 6.3 millidalton mass differences (Marshall et al., 1998, cited supra). However, for mass labels that are differentiated from each other by the smallest mass differences, higher resolution may be necessary and this can currently be achieved routinely on commercially available Fourier Transform Ion Cyclotron Resonance mass spectrometers.

Time-of-Flight (TOF) mass spectrometers are a further example of a type of mass spectrometer from which high resolution, high mass accuracy data may be obtained depending on the length of the flight tube. Commercially available, Multi-turn (Okumura, D. et al., (2005) Eur J Mass Spectrom (Chichester, Eng), 11, 261-266) and Spiral TOF (Shimma, S. et al., (2012) PLoS One, 7, e37107) geometries can already achieve mass resolution similar to Orbitraps.

The Orbitrap mass spectrometer consists of an outer barrel-like electrode and a coaxial inner spindle-like electrode that form an electrostatic field with quadro-logarithmic potential distribution (Hu, Q. et al., (2005) J Mass Spectrom, 40, 430-443 & Makarov, A. (2000) Anal Chem, 72, 1156-1162). Image currents from dynamically trapped ions are detected, digitized and converted using Fourier transforms into frequency domain data and then into mass spectra. Ions are injected into the Orbitrap, where they settle into orbital pathways around the inner electrode. The frequencies of the orbital oscillations around the inner electrode are recorded as image currents to which Fourier Transform algorithms can be applied to convert the frequency domain signals into mass spectra with very high resolutions.

In Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometry, a sample of ions is retained within a cavity like and ion trap but in FTICR MS the ions are trapped in a high vacuum chamber by crossed electric and magnetic fields (Marshall, A. G. et al., (1998) Mass Spectrom Rev, 17, 1-35 & Marshall, A. G. and Hendrickson, C. L. (2008) Annu Rev Anal Chem (Palo Alto Calif.), 1, 579-599). The electric field is generated by a pair of plate electrodes that form two sides of a box. The box is contained in the field of a superconducting magnet which in conjunction with the two plates, the trapping plates, constrain injected ions to a circular trajectory between the trapping plates, perpendicular to the applied magnetic field. The ions are excited to larger orbits by applying a radio-frequency pulse to two ‘transmitter plates’, which form two further opposing sides of the box. The cycloidal motion of the ions generate corresponding electric fields in the remaining two opposing sides of the box which comprise the ‘receiver plates’. The excitation pulses excite ions to larger orbits which decay as the coherent motions of the ions is lost through collisions. The corresponding signals detected by the receiver plates are converted to a mass spectrum by Fourier Transform (FT) analysis. The mass resolution of FTICR instruments increases with the strength of the applied magnetic field and very high resolution (>1,000,000) analysis can be achieved (Schaub, T. M. et al., (2008) Anal Chem, 80, 3985-3990).

For induced fragmentation experiments, FTICR instruments can perform in a similar manner to an ion trap—all ions except a single species of interest can be ejected from the FTICR cavity. A collision gas can be introduced into the FTICR cavity and fragmentation can be induced. The fragment ions can be subsequently analysed. Generally fragmentation products and bath gas combine to give poor resolution if analysed by FT analysis of signals detected by the ‘receiver plates’, however the fragment ions can be ejected from the cavity and analysed in a tandem configuration with a quadrupole or Time-of-Flight instrument, for example.

In a time-of-flight mass spectrometer, pulses of ions with a narrow distribution of kinetic energy are caused to enter a field-free drift region. In the drift region of the instrument, ions with different mass-to-charge ratios in each pulse travel with different velocities and therefore arrive at an ion detector positioned at the end of the drift region at different times. The length of the drift region determines mass resolution of TOF instruments and this may be readily increased. The analogue signal generated by the detector in response to arriving ions is immediately digitised by a time-to-digital converter. Measurement of the ion flight-time determines mass-to-charge ratio of each arriving ion. There are a number of different designs for time of flight instruments. The design is determined to some extent by the nature of the ion source. In Matrix Assisted Laser Desorption Ionisation Time-of-Flight (MALDI TOF) mass spectrometry pulses of ions are generated by laser excitation of sample material crystallized on a metal target. These pulses form at one end of the flight tube from which they are accelerated.

In order to acquire a mass spectrum from an electrospray ion source, an orthogonal axis TOF (oaTOF) geometry is used. Pulses of ions, generated in the electrospray ion source, are sampled from a continuous stream by a ‘pusher’ plate. The pusher plate injects ions into the Time-Of-Flight mass analyser by the use of a transient potential difference that accelerates ions from the source into the orthogonally positioned flight tube. The flight times from the pusher plate to the detector are recorded to produce a histogram of the number of ion arrivals against mass-to-charge ratio. This data is recorded digitally using a time-to-digital converter.

For the purposes of resolving all of the possible tags of this invention, mass spectrometers with high resolution are required but the nature of the instruments is not particularly important to the practice of this invention. In addition, many of the tags that have been described in this application can still be resolved on instruments with only single dalton resolution as long as subsets of the possible tags that are separated by single dalton mass differences are used.

The invention is detailed below by means of the following examples, which are merely illustrative and by no means limiting the scope of the invention.

EXAMPLES Syntheses of Mass Labels Example 1: Synthesis of 2-propyl-pyrrolidine-1-yl acetic acid

FIG. 1 illustrates a schematic of the 2-propyl-pyrrolidine-1-yl acetic acid reporter of this invention. Bromoethane is converted to a Grignard reagent by reaction with magnesium in ethoxyethane to give product 1. In parallel proline is protected at the amino group with a CBz protecting group to give product 2. The proline carboxylic acid is then reacted with the Weinreb amine to give the corresponding amide (3). This can be reacted with product 1 to give the ketone product 4. The CBz group is removed by reduction with hydrogen using a palladium/charcoal catalyst in methanol to give product 5. The ketone is then reduced with Lithium Aluminium Hydride to give 2 propyl-pyrrolidine (product 6). 2 propyl-pyrrolidine is then reacted with bromoacetic acid benzyl ester to give product 7. The benzyl protecting group is then removed by reduction with hydrogen using a palladium/charcoal catalyst in methanol to give product 8 which is the desired 2-propyl-pyrrolidine-1-yl acetic acid reporter. 

The invention claimed is:
 1. A set of two or more mass labels, wherein each mass label has the following formula:

wherein X is a reporter moiety having an exact mass and wherein the reporter moiety X has the following general formula:

wherein the dotted line indicates linkage to the rest of the compound, wherein * represents that oxygen is ¹⁸O carbon is ¹³C, nitrogen is ¹⁵N or hydrogen is ²H, and wherein one or more * are present, wherein each mass label in the set has an integer mass, wherein each mass label in the set has the same integer mass, wherein the set comprises two or more subsets of mass labels, each subset comprising one, two or more mass labels, wherein, when the subset comprises two or more mass labels, the exact mass of the reporter moiety X of each mass label in the subset is different from the exact mass of the reporter moiety X of the mass labels in the same subset and in all other subsets, wherein each mass label is distinguishable by mass spectrometry.
 2. The set of two or more mass labels according to claim 1, wherein each mass label has the following structure:

wherein * represents that oxygen is ¹⁸O, carbon is ¹³C, nitrogen is ¹⁵N or hydrogen is ²H, and wherein one or more * are present.
 3. The set of two or more mass labels according to claim 2, wherein * is ¹³C or ¹⁵N and the set comprises n=18 mass labels having the following structures:


4. The set of two or more mass labels according to claim 1, wherein each mass label has the following structure:

wherein * represents that oxygen is ¹⁸O, carbon is ¹³C, nitrogen is ¹⁵N or hydrogen is ²H, and wherein one or more * are present.
 5. The set of two or more mass labels according to claim 4, wherein * is ¹³C or ¹⁵N and the set comprises n=16 mass labels having the following structures:


6. The set of two or more mass labels according to claim 1, wherein each mass label has the following structure:

wherein * represents that oxygen is ¹⁸O, carbon is ¹³C, nitrogen is or hydrogen is ²H, and wherein one or more * are present.
 7. The set of two or more mass labels according to claim 6, wherein * is ¹³C or ¹⁵N and the set comprises n=18 mass labels having the following structures:


8. The set of two or more mass labels according to claim 1, wherein each mass label has the following structure:

wherein * represents that oxygen is ¹⁸O, carbon is ¹³C, nitrogen is ¹⁵N or hydrogen is ²H, and wherein one or more * are present.
 9. The set of two or more mass labels according to claim 8, wherein * is ¹³C or ¹⁵N and the set comprises n=7 mass labels having the following structures:


10. An array of mass labels, comprising two or more sets of mass labels as defined in claim
 1. 11. The array of mass labels according to claim 10, wherein the integer mass of each of the mass labels of any one set in the array is different from the integer mass of each of the mass labels of every other set in the array. 